The main downside to low coolant temps is your heater won't heat very well. Low temps can also effect the wear of critical engine parts over time as they are designed to operate at around 180 degrees, but this is a very low risk IMO. It can also decrease gas mileage slightly.

I'm guessing the cause is your thermostat is stuck in the open position. The thermostat is usually a really easy part to remove and check, and inexpensive to replace.

Remove it when the engine is cool and it should be in the closed position, if not its stuck in the open position. To further test the thermostat put it in c pan of water on the stove and heat the water, you should be able to see the thermostat open at around 180 deg, and when you turn the stove off and the water cools you should see it close again.

After goggling around abit it seems this is a common problem on Masda 3's and a new thermostat is the fix. It appears that the thermostat on these cars is in a tighter than normal spot and may require removing the idler pulley to get easier access to it, although many claimed to be able to change it without doing so using an 8mm wobble(universal) socket.

My wifes 02 Monte Carlo SS throws this code 3 or 4 times during the Winter months. Always in the morning when the temps are in the teens or less and I haven't let the car warm up before leaving. The car gets warm eventually, but by that time I'm already at work. I clear the code with my OBD-II and forget about it.

Depending upon how cold it is, you may be running in open loop mode. A lot of the sensor feedback is ignored, and you lose a lot of the advanced ignition, fuel mix, etc controls, so yes, it's something you want to fix (also why it impacts mileage).

To help raise low coolant temps, your PCM is gonna command your injectors to stay open longer (drop more fuel) to build up heat.

Negative ghost-rider. Engines run hotter when they are running a fuel-lean condition, not a fuel-rich condition.

Engines fail emissions when they run too cool because they can not get the cat(s) up to temperature. Depending on the type of catalyst, low temperatures can actually increase harmful emissions. Also, for maximum efficiency you need a lot of parts in your engine need to heat and expand to create a proper seal.

Negative ghost-rider. Engines run hotter when they are running a fuel-lean condition, not a fuel-rich condition.

Completely understand what you're saying, as this method (add air to fire to make it hotter) is used in cars with AIR injection to purposely pump more oxygen into the exhaust at start-up to heat up the O2's and CAT(S) faster.

But on a cold/cooler engine than normal (like OP's), I'm certain the injectors will open longer to drop more fuel to raise temps. Idle air/throttle is probably gonna be increased ofcourse to bring enough air to mix with the extra fuel, but the PCM is gonna command a rich mixture to raise temps, thus gas mileage might take a (small?) poop.

Completely understand what you're saying, as this method (add air to fire to make it hotter) is used in cars with AIR injection to purposely pump more oxygen into the exhaust at start-up to heat up the O2's and CAT(S) faster.

But on a cold/cooler engine than normal (like OP's), I'm certain the injectors will open longer to drop more fuel to raise temps. Idle air/throttle is probably gonna be increased ofcourse to bring enough air to mix with the extra fuel, but the PCM is gonna command a rich mixture to raise temps, thus gas mileage might take a (small?) poop.

Sorry, that is still incorrect. An engine running rich is cooler than an engine running lean in pretty much every situation. I've been doing PCM tuning for at least long enough to know that

Generally cold starts are often rich because the fuel atomizes poorly when cold making a lean start harder, however this depends on the motor, for example I could start my Sky RL relatively lean because it was DI and the atomization was much more controlled than a port injection car. You also run richer in open-loop (start-up) because you're not getting accurate feedback from the O2 sensors until they are hot (narrow band O2s are inaccurate until warmed up) and then switch to closed-loop and hunt for stoich.

So, yes, you cold start the motor rich because the goal isn't "get hot as quickly as possible" it is "make the motor start despite poor fuel atomization."

1) Computer sees the engine not up to temp yet, so it stays in open loop mode
2) Open loop means it ignores the O2 sensors, so it runs rich to be sure it won't blow something up by going too lean.
3) This extra fuel means your mpg drops (how badly it drops depends on the vehicle, some can drop as much as 5mpg).
4) Emissions go up and over time can also lead to plugging up the catalytic converter, fouling the O2 sensors, etc.
5) Plugged cat leads to even worse vehicle performance and even worse mpg

All this bad stuff can be caused by a simple thermostat sticking open.

1) Computer sees the engine not up to temp yet, so it stays in open loop mode
2) Open loop means it ignores the O2 sensors, so it runs rich to be sure it won't blow something up by going too lean.
3) This extra fuel means your mpg drops (how badly it drops depends on the vehicle, some can drop as much as 5mpg).
4) Emissions go up and over time can also lead to plugging up the catalytic converter, fouling the O2 sensors, etc.
5) Plugged cat leads to even worse vehicle performance and even worse mpg

All this bad stuff can be caused by a simple thermostat sticking open.

So, yeah, don't let it go too long.

Even with a jammed thermostat I have never seen an engine stay that cold while operating for any reasonable period of time, but this scenario is certainly plausible. I guess the climate down here just isn't cold enough. Good call.